Automatic volume control



Dec. 26, 1933. H METZ 1,940,874

AUTOMATI C VOLUME CONTROL Filed Feb. 9, 1932 Ki 5) E 2. B x 0 35 x 4 A 3{a 2 Q Signal Voltage.

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WITNESSES. INVENTOR Henry I Metz. W BY ATTORN EY Patented Dec. 26, 1933l I 1340,1114- I I AUTOMATIC VOLUME CONTROL Henry I. Metz,GhicopeeFallslMasa, assignor to. Westinghouse Electric -& Mann!su tor-inGompuny, a corporation of Pennsylvania; v Applicationl ebruary 9, 1932."Serial so. 591314 4 Claimsi (Cl. its-"44) My invention relates to volumecontrol appa ratus and'particularly to apparatusdesigned to compress thewide range of volume levels encountered in music; and speech before'they are im- I pressed upon a radiotransmitter or uponrecordingapparatus. 5 I

The range in volume levels for speech and music-is sogreat that itis'undesirableitoimpress energy having the same variation in amplitudedirectly uponradio-transmitter or recording apparatus. If the volume istoo great, the apparatus. is overloaded and distortion results. volumeis too low, the signals willbe transmitted inefficiently by a radiotransmittery' also, the

noise toisignal ratio will be too great.

Formerly, the volume has been controlled manually by: an operatorlocatedin the control room of the broadcast or recordingstudio.

-It is anobjectof my invention to effect volume control of. theabove-mentioned ty-peby means of I apparatus which automaticallycontrolsthe volume..,

'It is: a further object" of my inventiongto .so automatically controlthe volume. that the relative efiects of-soft and loud signals will.be'urr- It ;is:a .still-furtherobject of my invention to provideapparatus Lfor preventingcmore than 100 modulation. v It is ai-stillIurther'object of my invention to provide apparatus of theabove-mentioned type which: iscomparatively inexpensive and compact indesign. v

Inpracticing my invention, I impress the speech 35, ormusic' uponth'einput circuit of an unbalanced Wheatstone bridgei having resistancearms, the amplitude of the current in the output circuit of the bridgedepending upon the: amount the bridgexisunbalanced. The degree ofunbalance Q is controlled by means of a variable resistor in one arm ofthe bridge; thevalue oftheresistor being determined by the amplitude ofthe signal .current in the output circuit.

In one modification of my invention, the vari.- -15 able resistor isheated by current having. a value proportional to the output current ofthe bridge, .the resistor being of material such as tungsten "so that:its resistance increases with an increase in temperature'; v

v another modification of my invention-,de-

signed to prevent more than 100%; modulation, the variable resistorcomprises a pair of photo-v electric cells. The resistance of the-cellsis: determined by a beam of light. the intensityof 5 which: varies inaccordance with theoutput'current of the bridge. H r Y I V Otherfeatures and advantages of my invention will appear from the followingdescription taken in connection' with the accompanying" drawing,

in which: I I

If the Figure 1 is a circuit; diagram of avolumecon trolsystemconstructed in accordance with invention, Y, l

5 Fig. 2. isa .curve showing a. characteristic oil a class B amplifieremployed in my, system.

Fig. 3 is-a curve showing the temperature-resistance characteristicofthe variable, resistor. in the bridge arm, y I

Fig.4 is acurveshowing how the. two. above: mentioned. characteristicsmay be combmed. to produce a straight-linecurve, and" Fig. 5 is acircuit diagram illustrating a modlfl' cation of my invention."

. Referring to Fig. 1, theapparatus comprises. a

-Wheatstone bridge 10. having four. resistance arms, a, 12;. c and d;In, the leit-handtarms a and bfthe resistance units 11 and 12' are con}-nected directly into thecircuit. In the righthand arms 0 and d,theresistanceis connected into the bridge through; transformers '7. and8..

The arm 0 comprisestlie transformer primary 13. connected. directly intothe bridge .-circuit'. .It's secondary winding 14 has-Ia resistance unit'15 connected across. it. I

The armdcomprises the. transformer, primary 16 connected directly intothe bridge. The. secondary. 17 of the transformer 8. is connected totworesistance units 18 and 19.. connected. in. Se.-

ries. The resistancelunits. 18 and119 may be the filaments of UK 199vacuum. tubes. The trans]- formers '7 and 8.preferably have cores of anironnickelalloy, such as I-Iipernik.. 4 I

Theinput circuitis connected. across one. dies: onal of the bridgethrougha transformer 20,; The output circuit comprising. conductors21vand 22 is connectedacrossthe otherqdiagonalofthe bridge.

' The output of thebridge is amplified andtrans: mitted toatelephoneline 6301'. directly to a radio transmitting stationor to recordingapparatus by meansof anamplifier tube 23 to which the conductors 21 and22 are connected.

, The conductors 21 and 22 are connected to the input circuit of theamplifier tube 23 through a transformer 24 a volume controlpotentiometer 2,5 and. a biasing battery 26., The output of the tubecomprise a portion of the usual filter apparatus. The amplifieritube 23-isfofthe three-electrode indirectly-heated cathode type. I

I The arm d of the bridge is normally given such a low resistance valuethat. the bridge/is unbalaimed to 'a large degree and a large proportionof the input energy appears in thebridge output '23 is connected to theline 63by means 01! a.coupling choke coil 21 a condenser'28 and a.trans-' former 29. The choke coil 30. and condenser 31 circuit. .If theincrease in volume in the input v circuit were too great, the increasein volume in the output circuit would also be too great if. the

unbalanceoi the bridge remained unchanged. In accordance with-myinvention, the bridge iscaused to become more nearly balanced as thevolume of the input energy increases. Therefore, the increase in volumein the output circuit is less than it would be otherwise.

The control of the bridge balance is accom plished by means of anamplifier which has its input circuit 32 connected to the output circuitof the bridge and its output circuit 33 connected to the bridge arm (1.The input of this amplifier is shown connected to the output circuit ofthe bridge through the transformer 34 which is connected to the bridgeoutput conductors 21 and 22.

The secondary of the transformer 34 is connected to the grid and cathodeof an amplifier tube 35 through a volume control potentiometer 36. Theplate of the amplifier tube 35 is connected to a source of positivepotential through a resistor 37 and to the input circuit of theamplifier tube 38 through a coupling condenser 39 and a variableresistor 40. The amplifier tube 38 is adjusted to operate as a class Bamplifier.

' A class B amplifier is defined as one which operates so that the poweroutput is proportional to thesquare of the excitation voltage. This isaccomplished by operating the amplifier tube with a negative grid biassuch that the plate current is almost zero with no excitation. Anexcitation grid voltage is applied such that essentially half sine-wavesof plate current are produced on the least negative half -cycle of thegrid voltage. The grid usually swings positive on the excitation peaks.

The point between the coupling condenser 39 and the variable resistor 40is connected through a choke coil 41 to a grid biasing resistor 42. Thebiasing resistor 42 is connected between ground and the mid-point on aresistor 43 connected across the filament 44.

The plate 45 of the amplifier tube 38 is connected to a source ofpositive potential through a circuit which may be traced from the plate45 through the conductor 46 through both the upper half of thetransformer winding 17 and the resistance unitl8 and the lower half ofthe transformer winding 1'? and the resistance unit 19 and through theconductor 9 and the choke coil 48 to the positive terminal of thefilter. A by-pass condenser49 is connected between the plate 45 andground.

The conductors 46 and 9 are connected to the mid-points of secondary 17and resistors 18 and 19, respectively, for two reasons. One reason isthat, by having the plate current of tube 38 flow through the winding 17in opposite directions, saturation of the Hipernik core is avoided. Theother reason is thatundesirable interaction between the control or platecurrent of tube 38 and the audio signal in the bridge circuit isprevented.

The rectifier-filter unit for supplying plate potential is ofconventional construction and comprises a transformer 50, a double waverectifier tube 51 and. a filter having series inductance coils 52shunted by condensers 53.

It will be understood from the foregoing description that as the signalvolume at the bridge input increases, the unbalance of the bridge isdecreased by means of the increased heating current supplied by theamplifier tube 38. One reason for operating the tube 38 as a class Bamplifier is that its average plate current increases with increases ingrid excitation. The tube 38 may be looked upon as a form of amplifier.

Another reason for operating amplifier tube 38 class B is that theresistance of the resistors 18 and 19 changes with current passedtherethrough in the manner shown in Fig. 3. 1 It is desired that theresistance of resistors 18 and 19 change in a straight line relationwith respect to the signal voltage across reactor 41 as shown in Fig. 4,to obtain a uniform change in volume with change in signal level. Thisoperation is obtained by so adjusting the linearity compensator resistor40 that the output current of tube 38 follows curve A of Fig. 2 wherethe plate current is plotted against the signal voltage across reactor41.

- The resistor 40 is effective in changing the plate current-signalvoltage curve of tube 38 for the reason that the grid draws current atthe peak excitation voltages. With the resistance of re-- sistor 40 setat zero, the plate current follows the curve C of Fig. 2. If itsresistance is set at a low value the plate current follows the curve B.A still further increase in the resistance of resistor 40 results in thecurve A having the proper shape to combine with the curve shown in Fig.3 to give the straight line curve of Fig. 4.

It is evident from the above description that the linearity compensatorresistor 40 determines how uniformly the volume control operates withchanges in signal level. The adjustment of the potentiometer 36, on theother hand, determines the amount that the signal volume is compressed.

The modified circuit shown in Fig. 5 is the same as that shown in Fig.1, except that the resistance units 18 and 19 have been replaced byphotoelectric cells controlled by a neon lamp. The cathodes 54 and 55 ofthe photo-electric cells 56 and 5'7, respectively, are connected to theterminals of the transformer secondary winding 17, while the anodes 58and 59 are connected together and connected to the mid-point of thetransformer winding 17 through a battery 60. The resistance of thephoto-electric cells 56 and 57 depends upon the intensity of the lightemitted from a neon lamp 61. The neon lamp'61 is connected betweenground and a point on a resistor 62 in the platecircuit of the vacuumtube 38.

If the current in the bridge output circuit increases, the current inthe resistor 62 increases, and the voltage impressed across the neonlamp 61 increases so that the lamp emits a more intense light. Thisincrease in light intensity decreases the resistance of thephoto-electric cells 56 and 57 and the resistance of the bridge arm d isdecreased to improve the balance of the bridge. It is to be understoodthat the bridge was unbalanced by making the resistance of the arm d toohigh.

In the case of the modification shown in Fig. 1, the bridge wasunbalanced by making the resistance of the arm d too low so that thebalance was improved by increasing the resistance of the resistors 18and 19.

It will be understood that this apparatus does not give an output havingconstant volume. Such action is not desired as the relative effects ofsoft and loud music would be lost. The apparatus merely reduces therange in the change of volume levels.

In the apparatus shown in Fig. 1, there is a certain time delay in thevolume control action since it takes the resistors 18 and 19 anappreciable time to heat up. This delay is desirable in certain cases toavoid distortion.

However, if it is desired to employ my inven tion for preventing morethan 100% modulation in recording or transmitting equipment, any delayin the volume control action should be avoided and a circuit like thatshown in Fig. 5 should be employed where theresponse of thephotoelectric cells 56 and 57 to the change in light intensity isrelatively instantaneous.

Various modifications may be made in my in vention without departingfrom the spirit and scope thereof, and I desire, therefore, that onlysuch limitations shall be placed thereon as are imposed by the prior artand are set forth in the I ing having a, resistance unit connectedthereacross, and means for heating said resistance unit in accordancewith the amplitude of the energy in said output circuit.

2. A volume control system comprising a Wheatstone bridge having inputand output circuits, a transformer having primary and secondarywindings, said primary winding being in one arm of said bridge, and saidsecondary winding having a resistanceunit connected thereacross, andmeans for heating said resistance unit in accordance with the amplitudeof the energy in said output circuit, said means including a vacuum tubeoutput circuit having one output terminal connected to the mid-point ofsaid secondary winding and having the other output terminal connected tothe sistance unit. v

3. In a system of, the volume control type, an amplifier having an inputcircuit responsive to pedance in the output circuit of said amplifier,the voltage drop across whichis adapted to be used to compensate forchanges in the volume level of the system, said impedance beingnonlinearover a portion of its impedance characteristic curve and meansforcompensating for this non-linearity to produce a linear voltageresponseacross said impedance.

4. In asystem of the volume control type, a amplifier having an inputcircuit responsive to changes in the volume level of the system, animpedance in the output circuit of said amplifier,

: the voltage drop across which is adapted to be used to compensate forchanges in the volume level of the system, said impedance beingnonlinear over a portion of its impedance characteristic curve andmeans, for compensating for this non-linearity to produce 'a linearvoltage response across saidimpedance, said'means comprising a resistorin the input circuit'of said amplifier, said amplifier being adjusted todraw grid current on the peak voltages of the input signal energy.

HENRY I. METZ.

mid-point of said rechanges in the volume level of the system, an im- 7

